long-term temporal and spatial fluctuations of soft bottom infaunal invertebrates associated with an...

Int. Revueges. Hydrobiol. I 78 1 1993 I 4 1 535-555

DON MAURER~, GEORGE ROBERTS ON^ and THOMAS GERLINGER~ Biology Department, California State University, Long Beach, 1250 Bellflower Boulevard, Califor-

nia 90840, USA * County Sanitation Districts of Orange County, California, Fountain Valley, CaWrnia 92708-7018, USA

Long-Term Temporal and Spatial Fluctuations of Soft Bottom Infaunal Invertebrates Associated with an Ocean Outfall

from the San Pedro Shelf, California

key words: long-term, benthic, community structure, ocean outfall, California Shelf

Abstract

Temporal (annual) and spatial fluctuations of soft-bottom invertebrates from 1977 to 1988 were reviewed and analyzed from an ocean outfall area on the San Pedro Shelf, California. Several dominant species which occurred throughout the monitoring period provide continuity between early sampling efforts and recent ones. Annual fluctuations in the mean number of species, abundance, diversity and biomass were generally conservative compared to spatial fluctuations. Temporal changes of note include a marked reduction in abundance and biomass between 1981 and 1984 and another decline in biomass in 1988. The former was associated with conditions of the 1982-1983 El Niiio, and the latter with a shift in population dominance from a bivalve to a polychaete.

Mean abundance and biomass were enhanced by the outfall without a significant reduction in number of species. The response of the biota to the outfall extended further upcoast (northwest) than downcoast.

Several hypotheses were posed and briefly examined. These include the effects of total suspended solids generated from the outfall, and the role of the "Three Little Pigs" (bivalve-Parvilucina tenuisculpta CARPENTER, ostracod-Euphilomedes carcharodonta (SMITH), polychaete-Capitella capitata (FABRICIUS)) in influencing benthic community structure. Finally, measures of infauna from the San Pedro Shelf were, except for abundance, very similar to those reported from other California shelf areas. Through time and space the outfall has had limited negative effect on infaunal community structure.

Contents 1. Introduction ................................................................... 536

a. Regionalgeology ............................................................ 536 b. Regional oceanography ....................................................... 536 c. Regional sediment geochemistry ..............................................

2. Methods and Materials ......................................................... 540 539

3. Results ....................................................................... 541 a. General .................................................................... 541 b. Number of species ........................................................... 541 c. Abundance ................................................................. 542 d. Biomass .................................................................... 544 e. Diversity ................................................................... 546

a. Species composition ......................................................... 548 b. Temporal fluctuations ........................................................ 548 c. Spatial fluctuations .......................................................... 549 d. Comparison of benthic community measures ................................... 549 e. Hypotheses ................................................................. 550

5. Summary ..................................................................... 553 6. Acknowledgements ............................................................. 553 7. References .................................................................... 553

.................................................................... 4. Discussion 548

536 D. MAURER et al.

1. Introduction

One ofthe goals of ecology is to determine the average and range of natural variability of abiotic and biotic factors to more accurately assess Optimum conditions of a Species pOpU- lation or multiple species populations (community). This assessment becomes very criti- cal when attempting to differentiate between the effects of natural processes and anthropogenic activities. Interactions between these two mechanisms make analyses and interpretations even more difficult. In view of the huge expenditure (billions of dollars) of funds in the United States towards pollution control, management decisions based on these ecological interactions are not inconsequential.

Long-term sampling is necessary to record the range of natural variability to evaluate interactions with anthropogenic activities (Estuaries, 1985; ESSINK, 1988). Marine monitoring programs (Baltic Sea, Wadden Sea, Southern California Bight, New York Bight) have become valuable sources of long term data (PEARSON, et al., 1985; ROSENBERG, et al., 1987; JOSEFSON & ROSENBERG, 1988; REISE & SCHUBERT, 1987; STULL, et al., 1986; STEIMLE, 1985). Such data sets also provide the basis for posing and testing new hypotheses in prepa- ration for making changes in monitoring goals and protocols (COULL, 1985; WOLFE, et af., 1987). The benthic monitoring program of the County Sanitation Districts of Orange County, California Districts, began in 1965 and has continued to the present. The Districts monitors its ocean outfall on the San Pedro shelf in the Southern California Bight. The purpose of this account is to describe long-term temporal and spatial fluctuations of the soft-bottom benthos.

Broad scale faunal surveys in the Southern California Bight include a variety of studies on the entire benthic fauna or major taxa (HARTMAN, 1955; BANDY, 1958; BARNARD, 1963; Allan Hancock Foundation, 1965; JONES, 1969; FAUCHALD and JONES, 1979; THOMPSON et af., 1987). Studies focusing on the Orange County ocean outfall include TURNER, et al., (1966), SMITH (1974), and MEARNS and GREENE (1975), and MAURER et al., (1990 a, b; 1993).

a. Regional geology

The Districts’ ocean outfall discharges 253 Million of Gallons (US) per day (9.58- los m3 d-l) (1988) oftreated (advanced primary and partial secondary) emuent onto the narrow San Pedro Shelf, California. The outfall is bounded on the west and east by the San Gabriel and Newport Canyons, respectively (Figure 1). Average winter waves control the offshore extent of the very fine sand on the central and eastern parts of the shelf (DRAKE, et al., 1985). Silt and clay temporarily deposited on the central shelf during the spring and summer months are resuspended and transferred to deeper areas in the winter.

Percent silt and clay is highest in the canyons, particularly along the Newport Canyon (Figure 2). In the western portion of the study area silt-clay progressively increases with increasing depth. The northeastern and north central nearshore area generally contains the smallest amount of silt-clay except for an area around the outfall which expands east- ward and northeastward towards Newport Canyon. In contrast the area immediately west of the outfall contains coarse sand and gravel probably exposed during outfall construction with admixtures of shell and plate fragments (bivalves & barnacles) associated with rock ballast and the diffuser pipe.

b. Regional oceanography

The physical oceanography of the San Pedro shelf involves the southward flowing Cali- fornia Current (CC), the northward flowing California Undercurrent (CU) and the Inshore

Soft Bottom Invertebrates from San Pedro Shelf

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537

Figure 1. Historical Station Locations, San Pedro Shelf, California (Districts).


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Figure 2. Representative Sediment Data-Mean Percent Silt and Clay, San Pedro Shelf, California (Districts 1985-1989).

Soft Bottom Invertebrates from San Pedro Shelf 539

Countercurrent (IC) together with tidal flows, internal waves and river flows (HICKEY, 1979; SIMPSON, et al., 1984). Seasonal shoaling of the CU and the fall-winter appearance of the IC together with storms, Fronts and eddys also interact with the study area (LYNN and SIMPSON, 1987).

Strong sediment transport processes occur in the shore zone. These flows are impeded by canyons which bound the Districts’ outfall serving as sediment traps. Bottom currents in the vicinity of the Districts’ outfall are predominately northward (upcoast). Shallow currents achieve speeds as high as 35 cm/s but longer term average speeds are generally lower (10-15 cm/s). Currents normally diminish through the winter quarter and increase again in the spring. Currents are strongly influenced by local bathymetry with the flow near the diffuser primarily aligned with the bottom contours (CSDOC, 1989).

Depending on the degree of mixing or stratification the annual range of water quality profile parameters encompasses: temperature 9-19 OC, salinity 33.2-33.9 %o, dissolved oxygen 2.9-11.3 mg/l, pH 7.1-8.6, transmissivity 26-89%, total suspended solids 0.1-15.6 mg/l, ammonia 0.02-0.24 mg/l, oil and grease 0.2-0.6 mg/l, and fecal coliforms 2-5,400 mpn/100 ml. For temperature, dissolved oxygen and pH discharge related trends are not noted outside the Zone ofInitial Dilution (ZID, an area 60 m in width on either side ofthe outfall diffuser) (CSDOC, 1989). In contrast salinity, total suspended solids, oil and grease, ammonia, fecal coliforms and light transmissivity exhibited some discharge related trends. However, many of these changes are restricted to the nearfield and to water depths within or below the thermocline, thereby reducing any potential water quality effects or transport of discharge plume constituents towards shore.

c. Regional sediment geochemistry

Total volatile solids (TVS) and total organic carbon (TOC) averaged 1.9Yo and 0.47%, respectively, and ranged from 0.77 to 2.8 % and from 0.42 to 0.50 %. Both parameters were highly associated with median sediment particle size. Chemical oxygen demand (COD) averaged 2 1 mg/g and varied from 14 to 26 mg/g. Dissolved sulfide ranged from 8.1 to 34 pg/g and averaged 15 pg/g. Cyanide (CN) concentrations ranged from 0.029 to 0.036 pg/g with a mean of 0.032 pg/g. Highest sulfide and CN values were recorded from Newport Canyon. Oil and grease measurements provide an indication of the levels of petroleum related hydrocarbons in bottom sediments. Concentrations of oil and grease ranged from I5 to 119 mg/l with a mean of 66 mg/l for 1988-89. Oil and grease concentrations show enrichment at the ZID and ZID boundary stations relative to concentrations at the Dis- tricts’ reference station. Ordination plots of oil and grease indicate trends related to the outfall (CSDOC, 1989).

The mean and range of concentrations of trace metals and selected organic constituents in sediments from 1988-89 (CSDOC, 1989) and the 1985 SCCWRP Reference Site Survey (THOMPSON, era/., 1987) are presented in Table 1. These values were generally similar with a few exceptions. Pb, Ni, Cr, and Zn are highly associated with each other and with TVS, TOC, median sediment size and % silt-clay (CSDOC, 1989). In contrast Ag and Cd are not well associated with grain size parameters, TVS or TOC. Distribution patterns observed suggest that the occurrence of Ag, Cd and Cu differs from that of the Cr, Ni, Pb and Zn group of metals. Moreover, the Ag-Cd-Cu group provides a more marked discharge related signature than the Pb-Ni-Cr-Zn group. The latter group is more associated with broader geological and physical oceanographic processes.

There were no consistent trends in the tPAH concentrations. Total DDT concentrations are relatively low with the higher values recorded in depositional areas, particularly in the canyons. Elevated levels of total PCB occurred in sediments near the outfall, but deposition of this constituent was episodic rather than persistent. Long-term trends in


Table 1. Mean concentrations and ranges of trace metals; total polynuclear aromatic hydrocarbons, total phenols, total DDT and total PCB in sediment. Data from the 60 m

CSDOC 1988-89 and 1985 SCCWRP Reference Site Survey stations. CSDOC SCCWRP 1988-89 n=39 1985 n=38

Parameter X Range X - -

Arsenic Beryllium Cadmium Chromium Copper Lead Mercury Nickel Selenium Silver Thallium Zinc tPAH Phenols Total DDT Total PCB

4.1 0.255 0.559

19.9 16.4 10.0 0.053 8.38 0.244 0.474 0.157

49.2 127 289

34 8.7

1.49 - 13.7 0.11 - 0.69 0.11 - 2.42 9.7 - 34.8 5.73 - 59.6 5.05 - 29.8 0.020- 0.49 4.49 - 19.0 0,lO -- 2.44 0.09 - 0.99 0.08 - 0.33

ND-1700 ND-3300

3.0 - 31 ND-118

24.8 -112

- 0.14

25.4 10.4 4.8

12.9 - -

0.03 - 48.0 20.3

18.9 19.2

-

n = Number of samples Inorganics = mg/kg dry wt. Organics = pg/g dry wt.

concentrations of total DDT und PCB are more related to regional sedimentation patterns rather than outfall discharge.

2. Methods and Materials

From 1977 through the present at least 20 annual (summer) stations (Figure 1) have been sampled with a modified Van Veen grab (0.1 m2) and the sediment has been sieved through a 1.0 mm mesh screen. Other stations have been added, but they are not part of the long-term database (CSDOC, 1989). Expanded taxonomic expertise between 1984 and 1985 contributed in part to increases in number of species and diversity for this period.

The 20 stations which have been sampled annually since 1977 include (Figure 1): nearshore (1 1,14, 15), transitional (6,7,8), within the ZID (0,4), upcoast nearfield (1,2,3,5), upcoast farfield (C, 10,13), downcoast nearfield (9,12) and offshore (17,18,19). Since these stations represent the longest con- tinuous source of information about the Districts’ benthic monitoring program, they are the focus of this account.

Infaunal samples were sorted under dissection microscopes in five major groups: mollusks, polychaetes, crustaceans, echinoderms and minor phyla. Organisms sorted from the samples were identified to the closest practicable taxonomic group and counted. Wet weight biomass was measured for each of the five major taxonomic groups. Basic measures of community structure which were estimated include: number of species/O.l m2, abundance (number of individuals/O.l m2), K, and wet weight biomass (gIO.1 m2). K is a measure of diversity as defined by PIELOU (1977).

Data from 1977 to 1988 were log (natural) transformed and treated with Bartlett’s test for homoge- neity ofvatiance. Transformed data were then treated with a one way analysis ofvariance for the time (year) and again for station. After this the Student-Newman-Keuls (S-N-K) Test and Tukey’s-proce- dures were applied. The analyses were performed using the computer statistical package SYSTAT (WILKINSEN, 1988).

Soft Bottom Invertebrates from San Pedro Shelf 54 1

3. Results

a. General

The soft-bottom infauna ofthe southern California shelf is very rich (a4500 macrofaunal species). Since the inception of this sampling program in 1965 to the present, approximately 1200 species have been recognized. For example, during the 1988-89 sampling period alone, 606 infaunal species were identified (CSDOC, 1989). Taxonomic listings can be found in the Districts’ annual reports.

b. Number of species

From 1977 through 1988 the mean number of species per station and year ranged from 79 to 102 and from 66 to 122, respectively. Mean values for stations are based on an Nof 12 (yrs), mean values for years are based on an Nof2O (stations). The mean number of species was generally higher at the control station (C), nearshore stations and transitional stations than at the offshore stations, near field stations and within ZID stations (Figure 3a). The relatively low mean number of species for Station 15 may reflect its position off the mouth of the Santa Ana River. ANOVA revealed that there was no statistically significant (a = 0.05) difference in the mean number of species among stations (Table 2).

Table 2. Results of ANOVA on Infaunal Parameters 1977 to 1988

Species Abundance Biomass K Count

Station F Value 0.88 11.6 2.2 11.1

Year F Value 32.0 4.4 2.6 8.1

Probability 0.60 0.000 0.003 0.000

0.000 - 0.000 0.003 0.000 Probability - Underlined = Statistically significant (0 = 0.05).

During the same period there was a progressive decline in the mean number of species to a low (66) in 1981 and an increase to a high (122) in 1988 (Figure 4a). ANOVA indicated that there was a significant difference in this parameter among yean (Table2). Based on several a posteriori tests (S-N-K and Tukey) the mean number of species estimated for 1985 through 1988 was significantly higher than all other years. The rapid increase of this parameter from 1984 to 1985 was partially due to increased taxonomic expertise (f: 15 %).

To provide an impression of change through time per station the mean number of species was plotted for several five year intervals (1977, 1982, 1987). During this period the number of species per station generally increased (Figure Sa). This trend was particularly noticeable between 1982 and 1987 and at the outfall and nearfield stations.

542 D. MAURER el a/.

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Mean Species Count (n= 12) with standard error bars per Station (No.IO.1 m2), San

Mean Abundance (n= 12) with standard error bars per Station (NoJO.1 m2), San Pedro Pedro Shelf, California (Districts 1977-1988).

Shelf, California (Districts 1977-1988).

c. Abundance

From 1977 through 1988 mean abundance (NoJO.1 m2) per station and year ranged from 336 to 1308, and from 450 to 830, respectively. Mean abundance was higher at the outfall, near field, and offshore stations than the near shore, transitional, far field and control stations (Figure 3b). As one approaches Station 0, along the 60 m stations, there is a rapid increase in mean abundance between Stations 5 and 0. In contrast, the rate of increase from the east between Stations 12 and 0 is more gradual. ANOVA indicated that mean abundance was significantly different among stations (Table 2).

S-N-K and Tukey analyses indicated that mean abundance was significantly higher at Station 0 compared to every other station. In addition, mean abundance at Stations 1,3, and 4 was significantly higher than at several shallow water, transitional, farfield and control stations.


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Mean Biomass (n= 12) per Station with standard error bars, San Pedro Shelf, California

Mean diversity K (n= 12) with standard error bars per Station, San Pedro Shelf, Cali- (Districts 1977-1988). Wet weight (g/O.1 mZ).

fornia (Districts 1977-1988).

Mean abundance increased from 1977 to a peak (~830) in 1979 followed by a decline to a low (~4450) in 1982 (Figure 4b). From 1982 to 1988 mean abundance has generally increased with noticeable annual fluctuations. The peak in 1979 was mainly influenced by the bivalves Parvilucina tenuisculpta (CARPENTER), Axinopsida serricata (CARPENTER), Tellina spp., the polychaetes Mediomastus californiensis HARTMAN and Pectinaria californiensis HARTMAN and the ostracod Euphilomedes carcharodonta (SMITH). ANOVA revealed that mean abundance was significantly different among years (Table 2). S-N-K and Tukey analyses showed that mean abundance in 1979 was significantly higher than in 1981, 1982 and 1984; mean abundance in 1988 was significantly higher than in 1981 through 1984 and 1986 and 1987, and 1985 was higher than in 1982.

Abundance was plotted for several five year intervals. During this period abundance remained the same or may have declined slightly from 1977 through 1982 (Figure 5b). From 1982 through 1987 the major increase was restricted to Station 0.

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Mean Species Count (11-20) with standard error bars per Year (NoJO.1 m2), San Pedro

Mean Abundance (n=20) with standard error bars per Year (No.lO.1 m2), San Pedro Shelf, California (Districts 1977-1988).

Shelf, California (Districts 1977-1988).

d. Biomass

From 1977 through 1988, mean biomass wet weight (g/O.1 m2) per station and year ranged from 5.2 to 14.2 and from 5.8 to 15.8, respectively. Mean biomass was generally higher at the outfall and nearfield stations than the nearshore (except for Station 15), transitional, control, farfield and offshore stations (Figure 3c). The relatively high mean biomass at Station 15 (14 g/O.l m2) was primarily due to the high abundance of ophiuroid echinoderms, principally Arnphiodia spp. This taxon was also represented by high numbers of juveniles. ANOVA revealed that there was a significant difference in mean biomass among stations (Table 2).

However, S-N-K and Tukey analyses showed that mean biomass was only significantly different between Station 2 and Station 14. There was a gradual increase in mean biomass


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from 1977 to the major peak in 1980 followed by a rapid decline in 1982 (Figure 4c). Mean biomass then increased to a secondary peak in 1983 followed by another decline to its low in 1987. The 1980 peak was primarily attributed to the bivalves P. tenuisculpta, A. serricata, Macorna spp., Tellina spp. and Lucinoma annulata (REEVE). The 1983 peak was mainly due to P. tenuisculpta, and Tellina carpenferi DALL. The inverse relationship between mean abundance and biomass in 1988 (Figure 4b and 4c) can be explained by a shift in populations between polychaetes and molluscs. ANOVA revealed that there was a significant difference in mean biomass among years (Table2). S-N-K and Tukey analyses showed that mean biomass in 1980 was significantly higher than in 1986 and 1987.

Biomass was plotted for several 5-Year intervals. Between 1977 and 1982, biomass levels were relatively consistent (Figure 5c). Between 1982 and 1987, biomass has markedly declined at all stations including the outfall except at Station 15. Increase in biomass here is attributed to ophiuroids; mainly Arnphiodiu spp.

546 D. MAURER et a/.

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Number of Species per Station at five year intervals (NoJO.1 m2), San Pedro Shelf,

Total Abundance per Station at five year intervals (NoJO.1 m2), San Pedro Shelf, Cali- California (Districts 1977, 1982, 1987).

fornia (Districts 1977, 1982, 1987).

e. Diversity, H'

From 1977 through 1988 mean H per station and year ranged from 2.8 to 4.0 and from 3.0 to 3.8, respectively. Mean H was lower at the outfall and nearfield stations than the nearshore, transitional, control and farfield stations (Figure 3d). ANOVA showed that there were significant differences in mean B among stations (Table 2). S-N-K and Tukey analyses indicated that mean H at Stations 0,1,3,4 was significantly lower than mean B at near shore, transitional, control and farfield stations. Since increased taxonomic expertise was evenly distributed throughout all stations from 1985 to 1988, these spatial trends are not influenced by any artifacts of taxonomy.

Mean H declined slightly from 1978 to 1979 and fluctuated moderately from 1979 with a slight increase through 1984 (Figure 4d). From 1984 through 1987 there was a relatively


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Biomass per Station at five year intervals, San Pedro Shelf, California (Districts 1977,

Diversity H' (n= 12) per Station at five year intervals, San Pedro Shelf, California

rapid increase in mean N. ANOVA showed that there were significant dmerences in mean H among years (Table 2). According to S-N-K and Tukey analyses mean IF from 1985 through 1988 was generally significantly higher than mean R for 1977 through 1984. In contrast to spatial patterns of mean N, temporal patterns of mean H are more influenced by artifacts of taxonomy than environmental processes. Annual fluctuations in K were slight to moderate at these stations. ff was plotted for several five year intervals. Between 1977 and 1982, R was very consistent across stations (Figure 5d). Between 1982 and 1987, H increased noticeably, particularly at nearfield and outfall stations.


4. Discussion

a. Species composition

SMITH’S (1974) pre-discharge and discharge study of the Districts’ 60 m outfall provides valuable information on past distributions of species composition. According to SMITH (1974) the ophiuroid Amphiodia urtica (LYMAN), P. tenuisculpta and Capitata (Mediomas- tus) ambiseta HARTMAN were abundant at the outfall during the pre-discharge period (1970-1971). After the outfall came on line, faunal density increased with a particular abundance of C. ambiseta. Coincidental with the increase of this polychaete, Amphiodia urtica declined in abundance near the outfall well into the eighties (a 1985). In the last cou- ple of years (1988 - present) this ophiuroid has begun to repopulate areas vacated throughout the seventies and mid eighties (CSDOC, 1989).

Such species as the polychaetes C. ambiseta, Pectinaria californiensis, Capitella capitata (FABRICIUS), Tharyx spp., Spiophanes missionensis HARTMAN, the bivalves Parvilucina tenuisculpta, Axinopsida serratica, Tellina spp., the ostracod Euphilomedes carchamdonta, and A. urtica were dominant (abundance, biomass, frequency of occurrence) at one time or another from 1965 to the present (MAURER, er al., 1990 b). All these species except one, occur widely and abundantly throughout the Southern California Shelf (JONES, 1969; FAUCHALD and JONES, 1979; JONES and THOMPSON, 1984). Capitella capitata occurs widely throughout the Bight, but is sporadically abundant (60,000/m2) in naturally enriched organic bottoms (HARTMAN, 1961, 1963) or ones associated with ocean outfalls (REISH, 1984). The above taxa have been recognized locally for almost 23 years and have consistently influenced benthic community structure throughout this period (MAURER, et al., 1990 b). Based on species composition alone the persistence of these dominant species for this period of time argues for relatively non-perturbed environmental conditions in and near the outfall area.

b. Temporal fluctuations

Annual and seasonal fluctuations in temperate benthic population dynamics are commonly very marked (FRANKENBERG and LIEPER, 1977; BUCHANAN, et al., 1978), and more recent studies are dispelling the former myth of temporal stability in tropical benthic communities (ALONGI, 1990). Since this present data focuses on annual fluctuations from summer time collections we can only address this time period; however, definite seasonal pul- ses in benthic community structure locally occur (CSDOC, 1989).

In contrast to characteristic marked annual fluctuations in benthos from temperate lati- tudes, changes in measures of local benthic community structure were very conservative from 1965 to 1977 (MAURER, et al., 1990 b) and more so from 1977 to 1988 (Figure 4). The increase in number of species between 1984 and 1985 was partially due to the infusion of taxonomic expertise associated with a new investigation team.

The 1983 El Niiio was one of the strongest such events ever recorded. This event affected oceanographic conditions and weather patterns throughout the Pacific including the California Shelf (Z~MMERMAN and ROBERTSON, 1985). Increased surface water tempe- ratures and nutrient depletion coincided with reduced phytoplankton and macrozoo- plankton biomass and surface chlorophyll levels (FIEDLER, et al.. 1986). In addition to nutrient depletion, winter storms also severly reduced kelp production (DAYTON and TEGNER, 1984; SEYMOUR, et al., 1989).

During the 1983 El Niiio depressed populations (abundance or biomass) were recorded by several workers throughout the Southern California Bight (MAURER and HAYDOCK, 1990; FERRARO, et al., 1991; THOMPSON, 1992). Moreover, the number of species, abun-

549 Soft Bottom Invertebrates from San Pedro Shelf

dance and biomass declined during 1981-1983 on the San Pedro Shelf (Figure4). We believe this temporary reduction was a part of this Bight wide response and deserves emphasizing because perturbations attributed to anthropogenic sources have commonly been confused with natural processes. Remission of the 1983 El Niiio conditions are associated with increased population estimates (MAURER and HAYDOCK, 1990; FERRARO, et al., 1991). The response of benthic organisms on the California Shelf to El Niiio may differ greatly due to oceanographic conditions elsewhere. For example, off Peru TARAZONA, et al. (1900 a, b) showed that benthos temporarily benefitted from an oxygen increase in areas normally hypoxic following the 1983 El Niiio. The elucidation of these natural processes are very important in accurately assessing the impacts of anthropogenic activities at particular sites.

c. Spatial fluctuations

According to the Pearson-Rosenberg Model (PRM) a progressive decline in number of species coincidental with a rapid increase in abundance and biomass may be expected towards an increasing gradient of organic enrichment (PEARSON and ROSENBERG, 1978). In extreme cases the number of species may yield to azoic conditions. From 1977 to 1988 there was a general trend of increasing abundance, and to a lesser extent biomass, towards the outfall (Figure 3b-3c). The number of species showed no relationship with the outfall for the same period. Abundance was the only community measure which was consistently significantly higher at the outfall station compared to other stations.

The PRM was examined with a 1985-1989, 13 station (60m) data set (MAURER, et al., 1993). By focusing on one isobath community variation with depth was eliminated from the analysis. Community variation with depth is well documented in the Southern Califor- nia Bight (THOMPSON, er al., 1987). Based on MAURER, er al.. (1993) it was determined that the number of species did not suffer at the expense of increased abundance at the outfall. Moreover, spatial fluctuations were not all attributable to outfall effects. For example, in the present analysis, the mean number of species, abundance and H was relatively low at Station 15 whereas biomass was relatively high (Figure 3). Station 15 occurs directly off the mouth of the Santa Ana River (Figure 1). Low number of species, abundance and H may be influenced by periodic freshets and increased rates of sedimentation from the river, while the high biomass is due to the ophiuroid Amphiodia spp. which may be benefiting from periodic inputs of sedimentation.

d. Comparison of benthic community measures

Comparison of benthic community measures from the Districts’ outfall with similar measures throughout the Bight provides some basis for assessing the degree of similarity or difference in community structure (Table 3). These data were all collected with the same gear (Van Veen) and were sieved through the same mesh size (1.0 mm) which makes the data very comparable. The Southern California Coastal Water Research Project (SCCWRP) provided the bulk of the data for comparison.

The Districts’ stations generally contain more species per station than other California sites with the exception of the SCCWRP Northern Reference site group (Table 3). In terms of number of species this measure of benthic community structure has not been changed by the outfall.

In contrast, abundance of benthos, particularly for the within ZID and nearfield stations, is approximately twice that at other California Shelf sites except for Palos Verdes (Table 3). Abundance has been significantly increased by the outfall.

D. MAURER et al. 550

Table 3. Benthic community parameters, Southern California Shelf sites 60 m stations

Districts Number Abundance Biomass Diversity Annual Survey 1977-1988 X Range X Range SZ Range SZ Range

of Species No/ m2 g/m2 H’

ZID

Nearfield 0,4 86 84-88 10,903 8,897-13,110 161 145-177 2.88 2.83-2.93

2. 3. 5. 1. 9. 12 83 79-86 7,435 5,968- 9,465 114 84-174 3.04 2.77-3.4 Fariieid ’ 13, 10 C

85 84-86 5,647 5,316- 5,978 95 93- 98 3.36 3.28-3.43 91 4,772 107 3.56

SCCWRP 1985 Reference Site Survey Northern Group 86 80-94 3,940 2,910- 5,770 152 70-181 3.5 3.3 -3.7 Southern Group 52 33-71 3,090 1,950- 5,510 81 14-149 2.8 1.9 -3.6

SCCWRP 1977 Reference Site Survey San Diego 47 2,280 66 3.1 South Santa Monica Bay 55 3,320 124 2.8 Hyperion Wuye 47 3,520 406 2.8 Palos Verdes 45 11,820 3 I9 2.0 Palos Verdes 59 16.4-100 8,067 2,920-40,850 234 63-616 - - (FERRARO, et a/., 1991)

Within the Districts’ area there is a suggestion of enhanced biomass in proximity to the outfall (Table 3). However, these estimates of standing crop are not the highest recorded for southern California. Moreover, the range of biomass estimated for the Districts is very similar to that recorded for the SCCWRP Northern Reference site group (Table 3). Bio- mass has been influenced by the outfall but not to the same extent as abundance.

Within the Districts’ area K was lower at the within ZID and nearfield stations than the farfield and control stations (Table3). The former values were more similar to the SCCWRP Southern Reference site group while the latter was more similar to the Northern Reference site group (Table 3). Mean values in number of species, biomass and K show considerable differences between Northern and Southern Reference site groups (Table 3). These values certainly suggest regional differences. Even within the Southern California Bight regional differences should be factored into comparisons between reference stations and test stations (i.e., outfalls) because they influence the interpretations of impacts.

e. Hypotheses

Long-term benthic studies also provide a basis for posing hypotheses (COULL, 1985; WOLFE, et al., 1987). MEARNS and WORD (1982) reported that average wet weight biomass of benthic invertebrates increased approximately proportional to mass emission rates of total suspended solids (TSS) at some southern California ocean outfalls. This relationship was expressed in terms of regression formulae relating standing crop and TSS. According to

55 1 Soft Bottom Invertebrates from San Pedro Shelf

this model (MEARNS and WORD, 19821, peak abundance and biomass should be associated with peak periods ofTSS. IfTSS from the Districts’ outfall increases, then abundance and biomass should increase. Conversely, reduction in TSS should yield reductions in abundance and biomass. The Districts’ outfall produced levels of TSS as high as 225,000 pounddday in 1978, decreasing to approximately 200,000 pounds/day in 1980, and to present levels of 100,OOO pounddday (Figure 6). From 1977 through 1983 the abundance- TSS relationship generally holds, and is consistent with MEARNS and WORD (1982) (Figure 6). However, during the period 1983 to 1985 TSS decreased rapidly, but mean abun-

250 1450 - 25 -

- 1250 -

* - 20

X

- - 5 450

250 - 0 1 1 1 1 1 1 1 1 1 1 1 ~ ~

77 78 79 80 81 112 83 84 Bs 86 87 at3 89 0

Year BIOMASS ........-... ABUNDANCE -- TSS

Figure 6. Total Suspended Solids (TSS) Concentrations (Ibs/day X Id), Abundance (Num./m*) and Biomass (glm’) per Year in the Districts‘ Discharge. San Pedro Shelf, California (Districts 1977-

1988).

dance increased to another peak in 1985, followed by a decline in 1986-1987 and another increase in 1988 (Figure 6). If the abundance-TSS relationship is an accurate predictor of the quantitative effect of increased organic enrichment as a stimulus for enhanced abundance, then abundance should have declined during the 1983 to 1988 period.

The relationship between biomass and TSS is less clear. Maximum mean biomass was recorded in 1980 (Figure 6) a year aRer TSS was reduced by 25,000 pounddday (CSDOC, 1989). Biomass progressively declined through 1982 during a period of relative stability for TSS (Figure6). From 1982 to 1983 biomass increased rapidly through a period of rapid reduction in TSS (Piyre6). This relationship is not consistent with the MEARNS and WORD (1982) paradigm. From 1983 through 1987 biomass once again declined during a period of rapid decline of TSS (1983-1985) followed by one of relative stability (lO0,OOO pounddday) of TSS (Figure 6). The hypothesis that the Districts’ outfall showed a simple quantitative relationship between biomass and abundance and TSS predicted by the MEARNS and WORD (1982) model was not supported here (Figure 6). We believe that the qualitative nature (type and concentration of contaminants) ofTSS also plays a role in this proposed relationship. The relative proportion of contaminants compared to carbon and nitrogen in,TSS deserve to be factored out in this analysis. Other regional sanitation districts (City of Los Angeles) are examining this relationship at their outfalls. Although the outfall exerts spatial effects in abundance and biomass, temporal change on an annual scale can also markedly effect the accuracy of these models. Natural events and

D. MAURER et al. 552

processes (El N ifio) independent of the outfall can influence Community stn~cture and function.

Increased abundance and biomass, coupled with reduced numbers Of Species, are Corn- monly considered characteristic of marine systems receiving anthropogenically derived organic enrichment (PEARSON and ROSENBERG, 1978; GRAY, 1989). These biotic responses have been interpreted as a system moving towards eutrophication (CEDERWALL a ELM- GREN, 1980; PERSSON, 1987). Presumably a few hardy, tolerant or opportunistic species effi- ciently exploit organic materials so as to reduce or exclude other species (REISH, 1984). A relatively diverse fauna with varied feeding types is replaced by a few dominant species of a more restrictive feeding type (deposit feeder). Tubificid oligochaetous worms in fresh water systems are classic examples. However, in the present case, the number of species was not significantly altered or reduced at the within ZID stations. Thus, a definitive characteristic of an organically enriched soft-bottom community is missing (MAURER, et al., 1993).

Examination of this data base revealed that a few species,-P. tenuisculpta, C. capitata and E. carcharodontu locally referred to as the “Three Little Pigs”, accounted for as much as 47% of the total number of infauna (MAURER, et a/., 1990 b). This enhancement did not preclude recruitment by other species. Since individual species were not measured for biomass, we cannot be quantitative in our evaluation, but P. tenuisculpta was clearly the big- gest contributor to biomass. However, depending on year and station, other bivalves and ophiuroids also contributed significant amounts to benthic standing crop. The contribution of the “Three Little Pigs” to local benthos is prevalent so that significant fluctuations in population levels of any one of these species, or some combination thereof, strongly influences measures of community structure (abundance, H and biomass). Other measures of community structure (dominance, evenness, species richness) presented elsewhere (CSDOC, 1989) are similarly influenced by the “Three Little Pigs”. The hypothesis posed here is that abundance, H and biomass are dominated by these three species.

PEARSON (1987) plotted the abundance ratio A:S (A is the total abundance per m2 and S is the total number of species per sample) for benthic stations (n= 1) on a transect through a sludge-disposal site in the Firth of Clyde, Scotland. The A:S represents the mean number of individuals per species and is expected to increase towards the center of the dump- site. The A:S (mean abundance/0.1/m2: mean number ofspecies) from the 1977-1988 historical data base was computed and plotted with and without the “Three Little Pigs” (Fi- gure 7). The A:S with all species, increased towards the within ZID stations (1,0,3,4). This increase is consistent with PEARSON’S (1987) projected abundance species ratio. The A:S without the “Three Little Pigs” also increased towards the within ZID stations, but the rate of increase was greatly reduced compared to the A:S with all species (Figure 7). This gra- phically shows how the “Three Little Pigs” contribute to total abundance and particularly influence abundance patterns at the within ZID stations.

Abundance and #from the 1977 to 1988 historical stations with and without the “Three Little Pigs” were compared. Based on a t-test, abundance was significantly (a = 0.05; t = 41.4) different. The biology of these three species can overwhelm measures of community structure as to unduly influence interpretations about the degree of apparent environmental stress. If these three species exclude recruitment by other soft-bottom invertebrates, reduce levels of secondary production, disrupt local food webs particularly those of demersal bottom feeding fish, increase disease susceptibility and become dominant bioconcen- trators of pollutants, then their influence on the benthos would be of serious concern.

BECKER (1988) has demonstrated how three species of flatfish in Puget Sound, Washing- ton which also occur locally, have preferentially fed on enhanced populations of Capitella spp. from disturbed areas. Enhanced populations of opportunistic prey have provided an additional source of nutrition to demersal predators. However, BECKER also points out that if these food rich areas attract and retain fish, trophic transfer of sediment contaminants,


20 cn 4 2 18

,$ 16

0

Near Shore Transitional Far Field Near Outfall Near Off Shore

Field Field E

t l i l I l l I i l i l l i l l l l l ~ l

I+ If 15 6 7 6 I C 13 10 5 1 0 4 9 12 3 is 18 17

553

Station

Figure 7. Mean Abundance per Station with and without the “Three Little Pig” San Pedro Shelf, - A : S Ratio-All Species - - A : S-Without “Three Little Pigs”

California (Districts 1977-1988).

susceptibility to disease and toxic effects of contaminants may be accelerated. Several of these conditions have not been locally met (species reduction, disease susceptibility) and disruption of demersal feeding habits remains to be tested.

5. Summary Examination of a 10 year record (1977-1988) of benthic invertebrates from the San

Pedro shelf, California showed considerable continuity. Annual fluctuations were very conservative except for one period (1981-1984) where natural processes (El Niiio) were probably dominant. Spatial patterns were also very consistent throughout this period with enhanced abundance and biomass associated with an ocean outfall.

Abundance and biomass were strongly influenced by three dominant species which have persisted during this period. The principal effect of the operation of a major ocean outfall for an extended period has been increased abundance and biomass without serious changes to community structure. Measures of community structure from the San Pedro shelf, except for abundance, were well within the range of other measures from both disturbed and reference shelf areas of California.

6. Acknowledgements

We are pleased to thank Dr. IRWIN HAYDOCK for his continued support and encouragement of our work and Mrs. ANNA UBALDINI who patiently helps prepare our manuscripts.

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